Boris Johnson will later publish plans to scrap parts of the post-Brexit deal he struck with the EU in 2020.
The PM wants to change the Northern Ireland Protocol to make it easier for some goods to move between Britain and Northern Ireland.
But the EU is against the move, saying it would break international law.
The UK insists its proposals - to be set out in a parliamentary bill - are legal and will secure the future of the United Kingdom.
The two sides agreed the protocol as they strived to maintain peace in Ireland - including the open Irish border - following Brexit.
Fifty two members of the 90 strong Northern Ireland Assembly, including those from Sinn Féin, the Alliance and the SDLP, have written to the prime minister to say they "reject in the strongest possible terms your government's reckless new protocol legislation". No unionist assembly members signed the letter.
Irish Prime Minister Micheál Martin has called for negotiations between the UK and EU to deal with the impasse.
He described the UK government's decision to change the protocol as a "low point", which was "very regrettable".
The Republic of Ireland is still part of the EU's single market for goods and services, which the UK has left.
Rather than impose a hard border - involving checks on goods and people moving between Northern Ireland and the Irish Republic - the protocol introduces checks on some goods travelling from Great Britain (England, Scotland and Wales) to Northern Ireland.
The Democratic Unionist Party (DUP), which won the second-most seats in the recent Northern Ireland Assembly elections, argues this creates a divide that could lead to the break-up of the UK.
It is refusing to set up a new ruling Northern Ireland executive with Sinn Féin, which won most seats in the elections and accepts the protocol, until changes are made to the wording.
There's been a long build up to this legislation which will, this week, generate a lot of noise.
The EU won't "sit by and watch the UK tear up a treaty", warned one Brussels diplomat.
The European Commission is expected to relaunch legal action against the UK, which was previously paused, once it's had a chance to analyse the bill.
That's seen as a first step or "low-hanging fruit".
But the commission is also likely, before long, to outline further "flexibilities" around its own proposals for easing checks on goods arriving into Northern Ireland from Great Britain.
It won't be anything like the more fundamental rewriting of the treaty that the UK's calling for but officials on both sides have signalled that they don't see today's legislation as ending any prospect of further talks.
In fact, some in Brussels view this bill, that's further exposed splits in a fractious Tory party, as a clumsy UK negotiating tactic.
There's speculation too that Liz Truss is trying to burnish her leadership credentials with some Conservative Brexit supporters.
But allies of the foreign secretary insist that, behind this potential law, there is a far simpler logic; if the EU won't renegotiate a treaty that is causing huge problems in Northern Ireland, then the UK government is obliged to act.
Putting questions around blame and motives aside, one clear outcome of today looks set to be a further deterioration in post-Brexit relations between the UK and the EU.
